It's crazy that organizations are willing to spend millions of dollars on Microsoft Office simply because people are used to it. There are literally no features most people actually use that aren't completely duplicated in open source alternatives. Whatever amount of time it takes the user to find the button they're looking for costs less than the permanent subscription cost for something that will only get more bloated and expensive with time.
Many times I've seen people state that they use Windows because they know it, but they can't do trivial things such as set up a printer or connect to WiFi.
Most user's Windows ability is to look for apps on the desktop or Start menu.
Shared contacts, calendars and coordination of meeting locations and virtual meetings.
I've yet to see FLOSS that matches that aspect of Outlook and o365/Exchange. I'm fact, IMO, it should have been one of the monetization efforts with Mozilla, which is a server companion for Thunderbird and a now comprehensive integration of calendar and contacts.
In terms of client software stuff that's true, but in terms of services it's not, as other comments here are pointing out.
It's the administration of user accounts, the certified compliance, the backups..
One thing that is missing that nowadays you get O365 so also management of employees access and licensing in single env.
You get backups, file synchronization, real time collaboration.
Setting and running all of that is as simple as making O365 account and clicking couple of buttons by one person.
There is no OSS solution that does that.
To replicate that with OSS you need 3 to 5 full time graybeards and it still will be annoying normal people that will not understand “why they can’t just do X as in MSFT tools”.